The present invention relates to a process for obtaining a polyisoprene having a high cis-1,4 linkage content from a steam-cracked C5 naphtha fraction enriched with isoprene.
Steam-cracked C5 naphtha fractions contain isoprene in a mass fraction of about 10%-30%. Such fractions can also contain:                mono-olefins (e.g. α-olefins and β-olefins), in a mass fraction of from 20% to 40%,        dienes, such as cyclopentadiene, 1,3-pentadiene (also referred to as piperylene) and other pentadienes, in a mass fraction of from 20% to 30%,        alkanes,        limonene (dimer of isoprene) and, in a minority proportion,        acetylene and aromatic compounds.        
In order to efficiently catalyze selective polymerization of isoprene with high activity from a steam-cracked C5 naphtha fraction, the fraction must first be enriched with isoprene so that the mass fraction of isoprene in the enriched fraction is close to 100%. In fact, it turns out that the presence of the other aforementioned compounds adversely affects the yield of the isoprene polymerization reaction. In particular, the enriched fraction must be practically devoid of cyclopentadiene, which is a poison to catalytic systems.
Enrichment of the mass fraction of isoprene in the steam-cracked C5 naphtha fraction may be obtained by implementing the following operations.
First and foremost, the cyclopentadiene is removed from the initial fraction using a polar solvent by fractional or extractive distillation, followed by distillation on maleic anhydride. To remove the acetylene compounds in the fraction, distillation may be performed over diisobutylaluminum hydride. Finally, the residual polar impurities are removed, for example, by passing over alumina.
It is known that polyisoprenes having a high cis-1,4 linkage content may be obtained using a catalytic system based on titanium tetrachloride and an alkylaluminum. The article by E. Schoenberg, H. A. Marsh, S. J. Walters, W. M. Saltman, Rubber Chemistry and Technology, 1979, vol. 52, pp. 564-565, indicates that, if this catalytic system is used for the polymerization of isoprene, the steam-cracked C5 naphtha fraction must have previously been enriched so as to comprise isoprene in a mass fraction of at least 97%.
Furthermore, Schoenberg et. al. teach that the mass fraction of mono-olefins in the enriched fraction cannot exceed 3.8% (of which the α-olefins cannot exceed 1% and the β-olefins cannot exceed 2.8%), and the limonene cannot exceed 0.1%. Moreover, the mass fraction of cyclopentadiene cannot exceed 1 ppm (ppm: parts per million), piperylene cannot exceed 80 ppm, and the acetylene compounds cannot exceed 50 ppm.
It is known that polyisoprenes having a high cis-1,4 linkage content may be obtained using catalytic systems based on:                a rare earth salt in solution in a hydrocarbon solvent,        an alkylating agent of this salt consisting of an alkylaluminum, and        an alkylaluminum halide.        
For example, the document “Report of the Academy of Sciences of the USSR, volume 234, no. 5, 1977 (Y. B. Monakov, Y. R. Bieshev, A. A. Berg, S. R. Rafikov)”, shows the use of a catalytic system comprising:                a bis(2-ethylhexyl)phosphoric acid salt of neodymium or praseodymium, as the rare earth salt, in solution in toluene,        triisobutylaluminum as the alkylating agent, in a molar ratio (alkylating agent:rare earth salt) of 20, and        diethylaluminum chloride as the alkylaluminum halide.        
Similar to catalytic systems based on titanium, the selective polymerization of isoprene with high activity can only be contemplated from a C5 fraction which has been enriched so that it will comprise isoprene in a mass fraction close to 100%.
The extraction from the C5 fraction of practically pure isoprene has the disadvantage of requiring the implementation of a complex separation process and, consequently, of involving relatively high operating costs for the polymerization of isoprene.